1. Field of the Invention
The present invention relates to an optical information read apparatus for optically reading information from a recording medium, and more particularly to a tracking control of the apparatus.
2. Description of the Prior Art
In recent years apparatuses for optically reading information from recording media such as video disk and digital audio disk have been extensively used. These media have microscopically narrow tracks for recording information. In order to read the information from the tracks, accurate tracking control is generally needed. For this purpose, any tracking error is required to be detected. Usually, optical means are utilized to satisfy this requirement. One of such optical means for detecting the tracking error uses the phase difference between two signals produced from an optical head as dislosed in Japanese Laid-Open Patent Application No. 52-93222 corresponding to U.S. Pat. No. 4,057,844. More specifically, an optical head includes a photodetector consisting of four photocells divided by two lines - one being parallel to the information recording tracks, and the other being perpendicular to the tracks. The output signals from the respective two diagonally disposed photocells are added to produce two detected signals. When a tracking error occurs, a phase difference is caused between these two detected signals. The phase difference is obtained by delaying the sum of the two detected signals by .pi./2 and then multiplying the delayed result by the difference between the two detected signals. An alternative to this multiplication method is disclosed in Japanese Laid-Open Patent Application No. 57-74837, in which the waveform of the sum of the two detected signals is shaped and then the difference between the two detected signals is sampled at the edges of the shaped waveform to obtain substantially the same result as the above-mentioned Dutch patent. Also, Japanese Laid-Open Patent Application No. 57-181433 discloses a method in which the waveforms of the two detected signals are shaped and the phase differences at the respective edges of the shaped waveforms are directly detected in digital technique.
However, in these conventional methods for detecting the tracking error, when a noise with a large amplitude due to a scratch or other defect on the recording medium is introduced into the output signals from the photodetector, the signal indicating the tracking error is accompanied by a large noise. This poses the problem that the tracking control is made unstable.